1. Field of the Invention
The present invention relates to the field of power dimmers for modulating the operating power of an essentially resistive load, for example, a lamp, powered by an a.c. voltage, for example, the mains voltage.
2. Discussion of the Related Art
FIG. 1 shows a basic diagram of a conventional light dimmer. Such a dimmer is most often based on the use of a triac 1 operating as a bidirectional switch, open in the idle state, connected in series with an incandescent or halogen lamp 2 between two a.c. supply terminals 4, 5.
Triac 1 is controlled by a circuit (CONTROL) 3, the function of which is to supply to gate G of triac 1 closing pulses at a frequency corresponding to double the frequency of supply voltage v. Circuit 3 is generally powered from the a.c. voltage by means of connections 6, 7, respectively, to terminal 4 and to the midpoint 8 of the series association of lamp 2 and of triac 1. Circuit 3 is associated with means (not shown), generally manually controlled, for setting the time of occurrence of the pulses with respect to the beginning of each halfwave of the supply voltage.
The operation of the power dimmer shown in FIG. 1 is illustrated by FIG. 2 which shows, in the form of a timing diagram, an example of the current i in lamp 2 and of control voltage v1 of triac 1. Signal v1 corresponds to a pulse train of amplitude V1.
In the absence of a triggering current in gate G of triac 1, the triac is blocked (non-conducting) and no current flows through lamp 2 (neglecting the supply current of circuit 3 which is very low due to the high resistance of lamp 2 and which causes no lighting thereof).
At the beginning of a mains halfwave (shown in dotted lines in FIG. 2), there is thus no current in the lamp. At a time t1, corresponding to the rising edge of a pulse P of signal v1, triac 1 turns on and current i in the lamp grows very rapidly to follow the mains sine wave. Triac 1 turns off at the end of the halfwave (time t2) and turns back on upon occurrence of the following pulse of signal v1.
A problem which arises in such a power dimmer is that the turning-on of the triac causes radioelectric disturbances on the mains, due to the current peaks which occur upon each closing of triac 1.
To solve this problem, an inductive and capacitive filter is generally used.
FIG. 3 shows an example of conventional power dimmer provided with such a filter.
An inductance L is interposed between lamp 2 and triac 1, to smooth the current peaks upon closing of triac 1. Inductance L is associated with a capacitor C connected in parallel on the series connection of inductance L and of triac 1, that is, connected between terminals 4 and 8. Capacitor C is used to limit the energy in inductance L. The association of inductance L and of capacitor C forms what is generally called a mains filter.
A disadvantage of using such a mains filter in a power dimmer is that inductance L is a particularly bulky and expensive component. Further, the inductance is not integrable.
Another disadvantage is that the inductance introduces an effect of lamp "flicker" when the dimmer is set at low power. Indeed, at low power, the hold current of the triac is higher than the current through the inductance, which causes oscillations upon switching of the triac.